Knowing soybean growth stages critical to rust management

With soybean rust making a late season push into soybean producing areas of the upper Southeast, knowing how to identify the growth stages of beans is critical to knowing when, what, or if to spray.

With the most recent rust find in southern North Carolina, it is clear soybean rust will have some impact on beans there, in Virginia and on up into the Delmarva Peninsula. How much of an impact remains to be seen, since a high percentage of the crop in these areas is past the growth stage that rust damages yield potential.

Soybean varieties as grouped in maturity groups (MG), starting with 00 varieties, which are commonly grown in southern Canada. At the other end of the spectrum, Maturity Group X beans are commonly grown in southern Texas. In between, maturity groups are often chosen based on time of planting, which may not always be the best management choice.

In the Carolinas and Virginia, a few growers have grown Group IV beans in an early planted, early harvest system, very similar time-wise to growing corn. In these systems, there is no threat from rust, because all these beans have been harvested, or are near harvest. A high percentage of Group V beans have either been harvested or are past the stage when rust is a threat and fungicides can legally be applied.

The beans at risk in the upper Southeast are primarily Group VI, Group VII, and Group VIII beans, planted late, often in a double-crop system.

Another criteria for applying fungicides may be yield potential of the crop. Many late planted beans don't have the 35 bushel per acre yield potential considered by many pathologists and economists alike to be the breaking point of when it is and isn't feasible to spray soybeans for Asian soybean rust.

In general, earlier Group V and higher varieties flower when nights are long and days are short. Conversely, Group III and Group VI beans flower when nights are short and days are long.

Group III and Group IV beans have an indeterminant growth habit, meaning they continue to add terminals to the main stem after the plant blooms. These varieties set a much higher crop load on the main stem.

For Group V and higher, terminal nodes are terminated at full bloom, but the plant adds buds on branches, providing for a much higher pod load on branches rather than the main stem of the plant.

On Group V and up beans, it is critical to get seeding rates right. Otherwise, the plant will be susceptible to lodging, which can open it up to diseases, like soybean rust. Seeding rates too high can also cause the plant to have a reduced ability to compete for nutrients, and subsequently to have under-developed pods and reduced yield.

Group IV and lower beans are highly sensitive to harvest. If left in the field too long, these varieties will be more susceptible to diseases, including soybean rust. However, all of these beans should be harvested in the upper Southeast, and should avoid rust problems in most years.

Group IV and lower beans may need some help in the Carolinas at harvest time. A pint or less of paraquat can be used to defoliate these beans, so they can be harvested as quickly as possible. With later-maturing beans harvest date isn't nearly so critical.

Because of their growth patterns, Group IV beans should be planted in the Carolinas from April 15 to May 10, Group V and Group VI should be planted from May 11 to June 10, and Group VII and Group VIII beans should be planted from June 11 to July 10.

Beans planted much later than mid-June have a higher risk factor for soybean rust and other disease problems.

Pawel Wiatrak, a plant pathologist at Clemson University's Edisto Agricultural Research and Education Center in Blackville, S.C., says that growers interested in planting Group IV soybeans should probably do so on a small acreage. If a grower can manage a small acreage of Group IV beans, this will give them a good idea of the management time and inputs required to grow these earlier beans on large acreages, he says.

Growth stages are critical to making good management decisions on spraying soybeans for rust and other diseases. When a grower sees an advisory to spray beans between Growth Stage 4 and 6, he or she better know how to identify these growth stages. A simple ride-by isn't likely to be a good indicator of when, or if, to apply fungicides to soybeans, according to Wiatrak.

When the first flower appears on the main stem on the majority of plants in a field, the plants, and for evaluation purposes the field, are in reproductive growth stage 1, or R1. When one flower on one of the top two nodes is full bloom, the plant is at R2, or reproductive growth stage 2. When pods on the top four nodes are less than one fourth -inch in length, the plant is in the R3 stage. When these pods reach one fourth inch or longer, the plant is at the R4 stage. At R5, seeds are formed, but are less than one eighth-inch in diameter.

By the time the plant reaches mid-R5 stage, and legally by R6, soybeans should not be sprayed for Asian soybean rust. At the R6 stage, seed are larger than one eighth-inch in diameter. At R7, one pod has the color of mature beans and at R8 95 percent of pods are mature.

Clemson University plant pathologist John Mueller says growers should always look at 20-25 plants in a field. If 50 percent of the plants fit into the defined growth stage categories, the grower should rate the field, he says. Looking at one of two plants isn't likely to give you a good evaluation of the growth stage of an entire field, and in larger fields with large differences in topography and soil quality, more than 20-25 samples may be needed to get an accurate picture of maturity.

Many areas of the Carolinas were hard hit by drought in the 2006 season. In these areas, soybeans frequently set a few pods during the drought, then after rains came later in the season, these plants flowered again. In these cases, Mueller says to go with the late bloom to make maturity evaluations, because the odds are better those blooms will set the most pods, he explains.

In 2005, rust came into soybeans first in South Carolina and spread quickly into North Carolina when the majority of beans were in the R3 to R4 growth stage. This year, rust was later developing, and so far has been at lower levels, not affecting most beans until they were in the R5 and R6 growth stage and beyond the point of yield loss risks from soybean rust.

For much of the 2006 season, rust was stuck in Georgia, and its spread into the Carolinas was delayed by 2-3 weeks. Last year, rust spread in about 50 mile increments into South Carolina, then in the same scenario into North Carolina. This year rust seemed to jump over 200 miles to South Carolina, possibly moved these greater distances by Tropical Storm Ernesto.

In addition to growth stages, growers should be aware of weather patterns when making decisions whether to spray for soybean rust. Typical weather in the Carolinas in mid-September to mid-October is 60-65 degrees F at night and 80-85 in the daytime. The ideal combination for soybean rust to spread is 65-85. If typical high and low temperatures are combined with cloudy, damp conditions, the environment is near perfect for rust to spread.

“Once soybeans are out of the R5 growth stage, there is no economic reason to spray for soybean rust. Growers who spray soybeans for rust, after Growth Stage 5 are not only losing money, they are breaking the law,” Mueller warns.

He says there should not be many growers with beans in the R1 or R2 stages, but for those who are in that situation, it may be wise to wait and see how rust develops before making the decision to spray.

Mueller says Asian soybean rust is a “wimpy little fungus”, and if the right fungicide is applied, the disease is easy to control. In South Carolina, he says many growers won't need to spray at all, and if a grower sprays more than once for soybean rust, they have probably sprayed too much, he contends.